L-gulonolactone oxidase is an enzyme that produces vitamin C, but is non-functional in humans and some other primates. It catalyzes the reaction of D-glucuronolactone with oxygen to L-xylo-hex-3-gulonolactone and hydrogen peroxide. It uses FAD as a cofactor. The L-xylo-hex-3-gulonolactone is able to spontaneously convert to hexuronic acid, without enzymatic action.